Generally, in a wireless communication, transmitted signals arrive at a receiver at different propagation times via a plurality of propagation paths, thus causing multipath fading. In order to prevent deterioration of transmission characteristics due to the multipath fading, a modulation/demodulation scheme having an anti-multipath characteristic is used.
The modulation/demodulation scheme having the anti-multipath characteristic includes, for example, a spectrum spread scheme, an OFDM (OFDM; Orthogonal Frequency Division Multiplexing) scheme in which information is distributed into a large number of subcarriers arranged over a wide range of frequency to be transmitted, and a so-called anti-multipath modulation scheme in which an anti-multipath characteristic is provided by adding phase redundancy or amplitude redundancy in a transmission symbol.
The spectrum spread scheme includes, for example, a DSSS (DSSS; Direct Sequence Spread Spectrum) scheme in which an original signal is multiplied by a spread signal with a band wider than the original signal, an FHSS (FHSS; Frequency Hopping Spread Spectrum) system in which a frequency is hopped over a wider band, and a THSS system (THSS; Time Hopping Spread Spectrum) in which a signal is spread by an impulse with a wide band.
The anti-multipath modulation scheme includes a PSK-VP (Phase Shift Keying with Varied Phase) scheme in which a convex phase redundancy is added (Non-patent Document 1), and a PSK-RZ (Return to Zero Phase Shift Keying) scheme in which the amplitude redundancy is added (Non-patent Document 2).
Additionally, even when a wireless communication is performed by use of a normal single carrier modulation scheme, the anti-multipath characteristic can be given by use of an equalizer at the receiving end. A modulation/demodulation scheme in which the wireless communication is performed by use of the single carrier modulation scheme, and the equalizer is used at the receiving end is also a modulation/demodulation scheme having the anti-multipath characteristic.
By using such a modulation/demodulation scheme with an anti-multipath property for communication, it is possible not only to prevent the deterioration of transmission characteristics due to a multipath waveform distortion, but also to actively improve the transmission characteristics with a plurality of delayed waves being received with diversity (path diversity) if there is an appropriate TDOA (time difference of arrival) between element waves forming the multipath (delayed waves) arriving at the receiver. Thus, with a path diversity, it is possible to obtain an effect of improving the transmission characteristics.
The appropriate minimum and maximum TDOAs with which a path diversity effect can be obtained will hereinafter be referred to as the “delay resolution” and the “maximum delay”, respectively. The delay resolution and the maximum delay may be determined based on the principle of the modulation/demodulation scheme used, or based on the parameters and/or limitations on implementation of the modulation/demodulation scheme.
For example, with the DSSS scheme, it is possible, on the receiver side, to separate a receive signal into delayed wave components and combine them together (RAKE reception) to obtain a path diversity effect, with a delay resolution corresponding to the 1-chip length of the spread code and a maximum delay corresponding to a value that is less than the spread code length.
With the OFDM scheme, the delayed wave components are absorbed at the guard interval set for the signal, whereby the maximum delay corresponds to the guard period. Inter symbol interference does not occur if the TDOA between delayed waves is within the guard interval. Moreover, since an error correction operation is normally performed over a plurality of subcarriers, information can be reproduced even if some subcarriers have errors therein due to a multipath distortion. The delay resolution corresponds to a value around the inverse of the frequency bandwidth. Thus, with the OFDM scheme, it is possible to obtain a path diversity effect (where the delay resolution is around the inverse of the frequency bandwidth) based on the effect of the guard interval and on the frequency diversity effect provided by scattering pieces of information over a wide frequency band and collecting the pieces together.
Moreover, when the PSK-VP scheme or the PSK-RZ scheme is used, the delay resolution corresponds to about several times less than a symbol length, and the maximum delay corresponds to a time less than 1 symbol-length. Furthermore, when the single carrier scheme, such as a PSK scheme and a QAM scheme is used at the transmitting end, the receiving end demodulates the signal utilizing an equalizer that uses a delay line with a tap. In this case, the delay resolution corresponds to 1 symbol-length, and the maximum delay corresponds to a time determined by the number of taps.
By artificially delaying the signal in performing a multi-station simultaneous transmission of the same signal from antennas of a plurality of base stations to thereby obtain the effect due to the path diversity by use of the modulation/demodulation scheme having the above-mentioned anti-multipath characteristic, there has been proposed a wireless transmission system which aims at active improvement of transmission characteristics, expansion of a communication area, or the like in a field of a cellular system or a broadcast system. Even in such a multi-station system, however, the effect due to the path diversity cannot be obtained at a point where the TDOA between incoming waves from respective antennas deviate from a range which is equal to or more than the above-mentioned delay resolution and equal to or more than above-mentioned maximum delay.
On the contrary, for example, when the TDOA between the incoming waves from two stations is extremely short, since the signals cancel each other at a point where two delayed waves with equal power are simultaneously received with opposite phases, the transmission characteristics will deteriorate significantly. Meanwhile, even in a point where the TDOA between the incoming waves from two stations exceeds the maximum delay, not only the path diversity effect is not obtained, but also the transmission characteristics will deteriorate. For that reason, in the conventional multi-station system, in order not to cause the above-mentioned problem, a proper difference is provided in transmission timings between the plurality of antennas for performing the multi-station simultaneous transmission, allowing the effect due to the path diversity to be certainly provided (for example, Patent Document 1).
FIG. 75A is a diagram illustrating a configuration of the conventional multi-station simultaneous transmission system described in Patent Document 1. In FIG. 75A, abase station 50 communicates with a moving terminal by use of a CDMA (Code Division Multiple Access) scheme. Remote antenna systems 52-1 through 52-n lie between the base station and the moving terminal which is not shown, and relay a signal transmitted between the base station and a mobile station. The remote antenna systems 52-1 through 52-n are arranged in a predetermined location far from the base station 50. The remote antenna systems 52-1 through 52-n include high gain antennas 54-1 through 54-n, delay elements 56-1 through 56-n, and remote antennas 58-1 through 58-n. 
The signals transmitted from the base station 50 are received by the high gain antennas 54-1 through 54-n to then amplified, are subsequently delayed by predetermined times at the delay elements 56-1 through 56-n, respectively, and are transmitted from the remote antennas 58-1 through 58-n. In this system, the delay elements 56-1 through 56-n that generate delay times which are multiple of a time (τ) little larger time than one chip time of the spread code and are mutually different are provided in the remote antenna systems 52-1 through 52-n, respectively. As a result of this, for example, when areas that respective remote antennas 58-1 through 58-5 cover are formed as E58-1 through E58-5 shown in FIG. 75B, the TDOA between the incoming waves at an area overlap point having approximately equal distances from adjacent local antennas, to which the signals arrive from the adjacent local antennas at approximately equal power is set to a proper value (in this case, approximately τ through 3τ), and thus making it possible to certainly obtain the path diversity due to the multi-station simultaneous transmission.
Meanwhile, in recent years, a multihop system in which a plurality of wireless stations have performed wireless communications by mutually relaying data has been studied. FIG. 76 is a diagram illustrating a configuration of the conventional wireless transmission system described in Patent Document 2. In FIG. 76, the wireless transmission system is provided with six wireless stations 17-1 through 17-6. FIG. 77 is a diagram schematically illustrating a transmission timing of a packet that each wireless station shown in FIG. 76 transmits.
First, the wireless station 17-1 transmits a packet for broadcasting. Only the wireless stations 17-2 and 17-3 that are located near the wireless station 17-1 can receive the packet that the wireless station 17-1 has transmitted. The wireless stations 17-2 and 17-3 wait for the transmission between a timing of completing reception of the packet and a predetermined transmission timing, and simultaneously transmit the packet.
Next, only the wireless stations 17-4 and 17-5 can receive the packets that the wireless stations 17-2 and 17-3 have transmitted. The wireless stations 17-4 and 17-5 also wait for the transmission from a timing of completing reception of the packets to a predetermined transmission timing, and simultaneously transmit the packets. The wireless station 17-6 then receives the packets that the wireless stations 17-4 and 17-5 have transmitted. As described above, according to Patent Document 2, an OFDM (OFDM; Orthogonal Frequency Division Multiplexing) scheme having the anti-multipath characteristic is used in the multihop system, and thus the interference is avoided even when the plurality of wireless stations simultaneously transmit the same packet. Moreover, since a time required for the packet transmission for broadcasting can be reduced as compared with a case where the multihop transmission is sequentially performed from the wireless stations 17-1, 17-2, 17-3, 17-4, 17-5, and 17-6, in this order, transmission efficiency can be improved.
As described above, according to the conventional wireless transmission system described in Patent Document 2, the plurality of wireless stations can efficiently perform the multihop transmission by use of the modulation/demodulation scheme having the anti-multipath characteristic.    [Patent Document 1] Japanese Patent No. 3325890 specification    [Patent Document 2] Japanese Unexamined Patent Publication (Kokai) No. 2000-115181    [Non-patent Document 1] H. Takai, “BER Performance of Anti-multipath Modulation Scheme PSK-VP and its Optimum Phase-Waveform”, IEEE, Trans. Veh. Technol., Vol. VT-42, 1993, November, p 625-639)    [Non-patent Document 2] S. Ariyavisitakul, S. Yoshida, F. Ikegami, K. Tanaka, T. Takeuchi, “A Power-efficient linear digital modulator and its application to an anti-multipath modulation PSK-RZ scheme”, Proceedings of IEEE Vehicular Technology Conference 1987, 1987, June, p 66-71